Excessive temperatures can degrade the performance of semiconductor devices. As an example, in an integrated circuit (“I”) chip, heat is generally generated through the operation of various integrated circuit devices on the chip. The heat from the individual IC devices is often conducted away from the IC devices to a heatsink. In a typical Personal Computer, heat from the heatsink may be transmitted to the air within a computer tower, for example, using fans to draw heat away from the semiconductor chip. Fans may then be used to remove heated air from the interior of the computer tower. Attempts have been made to improve the thermal conductivity at each stage in the heat removal process discussed above.
Specifically, at the semiconductor device level, various techniques have been developed to remove heat from the locality surrounding a semiconductor device. These efforts included making the semiconductor substrate very thin. For example some semiconductor substrates may be made about 25–50 microns thick. This thickness reduction may reduce the thermal resistance through the semiconductor substrate, which often is a poor thermal conductor, e.g. made of silicon or gallium arsenide. In addition, various attempts have been made to increase the thermal conductivity of the semiconductor material itself. For example, GaN, SiC, and other such semiconductor materials have relatively higher thermal conductivities, but have other drawbacks. In another technique, heat dissipators are spaced apart from each other on the surface of the semiconductor substrate, and in this manner improve the opportunity for heat to be conducted away from the substrate.
Another technique for improving the rate of removal of heat from a semiconductor heat source involves maximizing the thermal conductivity of the die attach medium (for example by using AuSn, and/or the like), and maximizing the thermal conductivity of the package base (for example, by using Cu, CuW, CuMo, and AlSiC materials, and less commonly, by using Be, BeO, Be—BeO, graphite, Cx (diamond), Silvar™, and/or like materials, or by using heatpipes or heatplanes).
Although these methods may be partially effective, it continues to be desirable to further improve the heat transfer rates at the semiconductor device level. For example, undesirably high thermal resistance exists in applications where heat sources (e.g., transistor gates or channels) have a small geometry relative to the chip, package, and heatsink geometry. In this exemplary case, the thermal resistance path from these small heat dissipating geometries to the ultimate heatsink may primarily include both conduction resistance and convection resistance.
The conduction resistance may be influenced by the solid medium beneath the dissipators, e.g. the geometries and conductivities of materials comprising the semiconductor, its attachment, and local packaging. Furthermore, in some packaged devices, a solid medium above the dissipators may also be included as part of the conduction resistance. For example, a solid medium above the dissipators may comprise the plastic in an overmolded package. Often, reduction of conduction resistance may be bounded by the use of practical semiconductor and packaging materials.
The conduction resistance may also include a component of spreading resistance. Spreading resistance may include the added thermal resistance encountered due to geometrical constrictions of heat flow. In the case of very small dissipators, e.g., a sub-micron power transistor gate located on the surface of a semiconductor substrate, the spreading resistance may be a significant contributor to undesirably high device temperatures.
If a device is not otherwise overmolded, as discussed above, the device may have air disposed above the semiconductor surface. In this example, heat may be transferred from the semiconductor heat dissipators to the air above them. Heat transfer through the air may include, for example, convection heat transfer. Convection heat transfer may be defined as the transfer of heat by the motion or mixing of molecules. Air, however, is a relatively good thermal insulator, and therefore resistance to this convection heat transfer is usually high relative to conduction resistances. In fact, convection resistance to air is generally high enough that the air is often ignored as a thermal path.
Thus, the ability to remove heat from an area near a heat generating device on an integrated circuit continues to be a limiting factor in the performance, reliability, and/or design capabilities of integrated circuit products. Furthermore, it is desirable to enhance the ability to remove heat from the small dissipating electronic feature geometries often found on semiconductor devices.